In certain chemical processing schemes, it is desirable for chemical reactions to take place in a reaction medium flowing in one or more relatively thin sheets. In such a processing scheme, the reaction progresses over an extended period of time while the sheets of reaction medium are exposed to the requisite reaction conditions. This type of process is particularly advantageous where the chemical reaction produces a gaseous reaction by-product, and it is desirable to rapidly and completely disengage such by-product from the reaction medium. For example, if the chemical reaction producing the gaseous by-product is reversible, failure to adequately disengage the by-product could counteract the desired reaction. When the reaction medium flows in one or more relatively thin sheets, the gaseous reaction by-product can rapidly escape the reaction medium. Further, when the reaction medium flows in one or more relatively thin sheets, the low hydrostatic pressure on the bottom portion of the reaction medium minimizes boiling suppression that can be exhibited when reactions are executed using relatively deep reaction mediums.
Although carrying out chemical reactions in relatively thin sheets of a reaction medium has a number of advantages, this type of process also presents a number of challenges. For example, because thin sheets of reaction medium require large amounts of surface area on which to flow, very large and/or numerous reactors may be required to produce commercial quantities of the reaction product. Further, in many processes employing thin sheets of reaction medium, the viscosity of the reaction medium changes as the reaction progresses. Thus, the viscosity of the final product may be much greater or much less than the viscosity of the initial reaction medium. This changing viscosity of the reaction medium presents a number of design challenges because significant variations in the flow rate and/or depth of the reaction medium can be undesirable.
One example of a common commercial process where it is desirable to carry out a chemical reaction in one or more relatively thin sheets of reaction medium is in the “finishing” stage of polyethylene terephthalate (PET) production. During the PET finishing stage, polycondensation causes the degree of polymerization of the reaction medium to increase significantly and also produces ethylene glycol, acetaldehyde, and water as reaction by-products. Typically, the degree of polymerization of the reaction medium introduced into the finishing reactor/zone is 20-60 while the degree of polymerization of the reaction medium/product exiting the finishing reaction is 80-200. This increase in the degree of polymerization of the reaction medium during finishing causes the viscosity of the reaction medium to increase significantly. In addition, since the polycondensation reaction associated with PET finishing is reversible, it is desirable to disengage the ethylene glycol reaction by-product from the reaction medium as quickly and completely as possible.
Thus, there exists a need for a more efficient and economical reactor that facilitates the processing of large quantities of a reaction medium in relatively thin sheets for extended periods of time. Further, there exists a need for a more efficient and effective PET finishing reactor that facilitates the polycondensation of large quantities of reaction medium flowing in relatively uniform, thin sheets through the finishing reactor, while providing adequate residence time to achieve the requisite degree of polymerization.